I hope it doesn't sound too arrogant, but there is no chance that you will be able to diagnose a car as I do, no matter what tools you use.
Do I think my tooling and experience are worth paying for? You bet!! But, you asked the question.
I do believe there is great benefit to being as knowledgeable as possible when dealing with maintenance and repair, even when taking it to me or any other tech. There are so many things to be decided in repairs. Read my comment about head work in another current post. The ability to solve the immediate problem is only part of the problem. Making a good economic judgment on repairs is more than a technical issue.
I doubt that there is good ROI (return on investment) for those with a trusted tech, but for those with curiosity and desire to know their car, these tools offer a window never before available to amateurs. The EASE people have used the capabilities for numerous markets. They sell to techs, DIYers, parents and fleet owners. They make another product that hooks to the OBDII daig socket that rats on your kids or professional drivers. It tracks mileage, rpms, speeds, time at rpm, time overheating etc., etc. Just about anything you could think to monitor.
They also make a more professional tool called a data logger that can monitor and log all values for hours. One could record a thousand mile trip and replay it on ones laptop, if one had such a desire.
Bosch Master, ASE Master, L1
33 years MB technician